In the production of mechanical pulps, such as TMP, CTMP, and RMP, it is necessary to refine the pulp sufficiently so that it can be used for its intended purposes. The degree of refining is commonly measured by measuring the "freeness" of the mechanical pulp. The freeness is a measure of how water drains from a pulp water mixture.
In conventional mechanical pulp producing systems, freeness is sometimes determined immediately after refining, and based upon the freeness measurements various refiner parameters are controlled to adjust the degree of refining provided by the refiner. For instance in some installations a conventional freeness measurement instrument, such as a DRT Freeness Measuring Instrument, available from Eur Control USA, Inc. of Decatur, Ga., is operatively disposed directly in the discharge line from the refiner (or the last refiner in the sequence of refiners). However accurate control of freeness does not ensue when utilizing such systems since the latent properties of the pulp interfere with the accurate measurement of freeness. In order to overcome this problem, a number of installations use conventional mixing and soaking chests, commonly referred to as "latency chests", to effect latency removal. Freeness is then measured after the latency removal. However since latency removal utilizing such procedures typically takes about 45 minutes to an hour, and because of this resulting delay, immediate control of the refiner is not possible. Therefore there can be substantial production of mechanical pulp that does not have the desired degree of refining.
According to the present invention, immediate and accurate feedback is provided so that the refiner can be controlled precisely to produce mechanical pulp having the desired degree of refining. This allows the production of a maximum amount of pulp having desired properties, and a minimum amount of pulp having undesired properties.
Immediate and accurate feedback control according to the invention is obtained by taking advantage of the virtually instantaneous removal of the latency of mechanical pulp that can be provided utilizing a fluidizing centrifugal pump, as disclosed in copending application Ser. No. 608,191 filed May 8, 1984, now U.S. Pat. No. 4,596,631, and the disclosure of which is hereby incorporated by reference herein. According to the present invention, shortly after refining (e.g. the last refining stage), the mechanical pulp is fluidized by a fluidizing centrifugal pump to effect removal of the mechanical pulp latent properties, and then a sample of the latency-removed pulp is selected and subjected to freeness measurement. The values of the freeness measurements, as well as other measured parameters such as consistency and flow measurements, are fed to a computer. The computer controls various refiner parameters in response to these measurements. For instance the computer controls the feed of comminuted cellulosic fibrous material to the refiner, and/or the hydraulic plate loading of the refiner (refining pressure) so as to adjust the degree of refining of the mechanical pulp produced. In this way mechanical pulp is produced having the desired degree of refining, as determined by the freeness of the pulp, in a quick and accurate manner. This maximizes the production of pulp having desired properties and minimizes the production of pulp with undesired properties.
It is the primary object of the present invention to provide a method and apparatus for the immediate accurate feedback control of a pulp refiner in the production of mechanical pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.